The invention concerns a new selective catalytic reduction of NOx, which is enabled by a side gas stream that can be separated from the main stream, or a side stream of hot air to provide for decomposition of the urea into its active components including ammonia.
Efforts are being made in many jurisdictions to reduce the emissions of nitrogen oxides (NOx). The technologies have included those that modify the combustion conditions and fuels, known as primary measures, and those that treat the exhaust after combustion, known as secondary measures. When effective primary measures are employed, the secondary measures can still be employed to achieve further reductions. To provide the best NOx reduction, it is apparent that both primary and secondary measures will be necessary.
Among the known secondary measures are selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). Both have been conducted with both ammonia and urea. See, for example U.S. Pat. No. 3,900,554, wherein Lyon discloses SNCR of nitrogen monoxide (NO) in a combustion effluent by injecting ammonia, specified ammonia precursors or their aqueous solutions into the effluent for mixing with the nitrogen monoxide at a temperature within the range of 1600° F. to 2000° F. Lyon also suggests the use of reducing agents, such as hydrogen or various hydrocarbons, to permit the effective use of ammonia at effluent temperatures as low as 1300° F. However, these temperatures are often too high for effective treatment, ammonia is difficult to deal with safely, and SNCR is not as effective as SCR. Similar processes are taught for urea by Arand, Muzio, and Sotter, in U.S. Pat. No. 4,208,386, and Arand, Muzio, and Teixeira, in U.S. Pat. No. 4,325,924. Again the temperatures are high, and the use of lower temperatures is not enabled.
SCR can operate with ammonia at lower temperatures, generally within the range of from 100° to 900° F. In this regard, see U.S. Pat. Nos. 3,032,387 and 3,599,427. SCR (selective catalytic reduction) has been available for years in some contexts for reducing NOx. To date, however, SCR has depended on the use of ammonia, which has safety problems associated with its storage, handling, and transport. Urea is safer, but has not been practical for many SCR applications due to the difficulty in converting it from a solid or an aqueous form to its active gaseous species that are reactive on catalyst bed for NOx reduction. Also, the reagent economics typically favor anhydrous ammonia over urea. In “A Selective Catalytic Reduction Of NOx from Diesel Engines Using Injection Of Urea” (Ph.D. thesis, September 1995), Hultennans describes a number of technical challenges in the context of Diesel engines while giving a broad background on the technology.
The use of catalysts for NOx reduction utilizing urea is effective but is sensitive to particulates and undecomposed urea, which can foul a catalyst. In this regard, it must be remembered that temperatures at the low end of the SCR treatment temperature range will not be high enough to fully gasify the urea. In addition, SCR requires very uniform mixing of active gaseous species prior to contact with the catalyst, and it is difficult to uniformly mix urea or its decomposition products with the large amounts of effluent in need of treatment. The limited attempts to use urea SCR for stationary and mobile sources, such as diesel engines, have been described in several recent patents including U.S. Pat. No. 5,431,893, to Hug, et al. To protect the catalyst from fouling, Hug, et al., proposes bulky equipment capable of treating all effluent with urea. Regardless of physical form, urea takes time to break down in hot exhaust gases and may cause nozzle plugging at the temperatures most conducive to gasification. This disclosure highlights the problems making it a necessity that the urea solution is maintained at a temperature below 100 C to prevent hydrolysis in the injection equipment. They propose the use of moderate urea pressures when feeding the urea and find it necessary to have alternative means to introduce high-pressure air into the feed line when it becomes plugged. The nozzles employed by Hug, et al., use auxiliary air to aid dispersion. Also, they employ dilute solutions that require significant heating to simply evaporate the water. See also, WO 97/01387 and European Patent Specification 487,886 A1.
In European Patent Specification 615,777 A1, there is described an apparatus that feeds solid urea into a channel containing exhaust gases, which are said to be hydrolyzed in the presence of a catalyst. For successful operation the disclosure indicates that it is necessary to employ a hydrolysis catalyst, compressed air for dispersion of fine solids, means for grinding the urea into fine solids and a coating to prevent urea prills from sticking together. The disclosure notes that if the inside of the catalyzer and the nozzle tip only were coated with the catalyst, corrosion and deposition would occur. Despite achieving the goal of removing water from the process, the specification introduces solid urea into the gas stream—possibly depositing urea on the SCR catalyst.
In U.S. Pat. No. 6,146,605 to Spokoyny, there is described a combined SCR/SNCR process in a staged process involving a separate step of hydrolyzing the urea prior to an SCR stage. A similar process is disclosed in U.S. Pat. Nos. 5,985,224 and 6,093,380 to Lagana, et al., which describe a method and apparatus involving the hydrolysis of urea followed by a separation of a gas phase from a liquid hydrolysate phase. Also, Copper, et al., disclosed a urea hydrolysis process to generate ammonia in U.S. Pat. No. 6,077,491. In all these processes there is a requirement to handle a significant amount of high temperature and high pressure gas and liquid phases containing ammonia during and after hydrolysis. This extra processing requires the purchase and maintenance of additional equipment, an emergency plan and equipment to handle ammonia release in case of process failures, and it would be desirable to have a system which operated more safely, simply and efficiently.
The art is awaiting the development of a process and apparatus that would permit the use of urea in an SCR process simply, reliably, economically, and safely.